X-ray generators are used to generate an electrical power pulse (i.e. an x-ray impulse) to cause an x-ray tube to emit x-rays in short bursts (impulses). In medical applications, there is increasing need for better well defined x-ray impulses. For example, with an increasing awareness of the risks associated with exposure to x-rays, it is highly desirable to limit patient x-ray exposure to the shortest time practical to achieve a desired x-ray diagnostic image.
As is well known, as the voltage across an x-ray tube increases during the leading edge of an x-ray impulse, at some threshold voltage, the tube begins to emit x-rays at an energy level, typically expressed in kilovolt electron volts (“keV”). At some voltage level of the leading edge of the x-ray impulse (related to the x-ray impulse rise time), typically at about 90% of the desired x-ray tube voltage, the x-ray radiation is useful and contributes to the desired exposure. Similarly, on the falling edge of the x-ray impulse (related to the x-ray impulse fall time), below about 90% of the desired x-ray tube voltage to about the threshold voltage, there is still x-ray emission, however at energy levels too low to be useful for the desired exposure. The x-rays emitted that have too low an energy to be useful to the desired diagnostic image, while they increase patient's radiation dose, are called “soft x-rays”.
“Crow-bar” circuits have been used in prior art x-ray apparatus to cause a faster falling edge of an x-ray impulse. A crow-bar circuit literally short circuits the output of the x-ray generator at the end of a desired x-ray impulse, thus more quickly discharging both the output capacitors of the x-ray generator and any charge stored in the distributed capacitance of the length of cable from the x-ray generator to the x-ray tube. Crow-bar circuits are both inefficient, because they waste any remaining stored energy as heat, as well as reduce the useful life of many x-ray generator electronic components since high peak currents associated with operation of a crow-bar are generally destructive to any components in the high current path.
Another problem is that a significant amount of stored energy in x-ray generator circuits is typically damped through discharge components and wasted as heat.
What is needed is an x-ray generator that can generate x-ray impulses having faster rise and fall times to minimize patient exposures to soft x-rays. What is also needed is a more efficient x-ray generator that can generate x-ray impulses having faster rise and fall times while minimizing wasted heat energy.